/*
 * Copyright 2002-2009 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.filter;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

import org.springframework.util.DigestUtils;
import org.springframework.util.FileCopyUtils;
import org.springframework.web.util.WebUtils;

/**
 * {@link javax.servlet.Filter} that generates an <code>ETag</code> value based on the content on the response. This
 * ETag is compared to the <code>If-None-Match</code> header of the request. If these headers are equal, the resonse
 * content is not sent, but rather a 304 "Not Modified" status.
 *
 * <p>Since the ETag is based on the response content, the response (or {@link org.springframework.web.servlet.View}) is
 * still rendered. As such, this filter only saves bandwidth, not server performance.
 *
 * @author Arjen Poutsma
 * @since 3.0
 */
public class ShallowEtagHeaderFilter extends OncePerRequestFilter {

	private static String HEADER_ETAG = "ETag";

	private static String HEADER_IF_NONE_MATCH = "If-None-Match";

	@Override
	protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {

		ShallowEtagResponseWrapper responseWrapper = new ShallowEtagResponseWrapper(response);
		filterChain.doFilter(request, responseWrapper);

		byte[] body = responseWrapper.toByteArray();
		String responseETag = generateETagHeaderValue(body);
		response.setHeader(HEADER_ETAG, responseETag);

		String requestETag = request.getHeader(HEADER_IF_NONE_MATCH);
		if (responseETag.equals(requestETag)) {
			if (logger.isTraceEnabled()) {
				logger.trace("ETag [" + responseETag + "] equal to If-None-Match, sending 304");
			}
			response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
		}
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("ETag [" + responseETag + "] not equal to If-None-Match [" + requestETag +
						"], sending normal response");
			}
			response.setContentLength(body.length);
			FileCopyUtils.copy(body, response.getOutputStream());
		}
	}

	/**
	 * Generate the ETag header value from the given response body byte array. <p>The default implementation generates an
	 * MD5 hash.
	 *
	 * @param bytes the response bdoy as byte array
	 * @return the ETag header value
	 * @see org.springframework.util.DigestUtils
	 */
	protected String generateETagHeaderValue(byte[] bytes) {
		StringBuilder builder = new StringBuilder("\"0");
		DigestUtils.appendMd5DigestAsHex(bytes, builder);
		builder.append('"');
		return builder.toString();
	}

	/**
	 * {@link HttpServletRequest} wrapper that buffers all content written to the {@linkplain #getOutputStream() output
	 * stream} and {@linkplain #getWriter() writer}, and allows this content to be retrieved via a {@link #toByteArray()
	 * byte array}.
	 */
	private static class ShallowEtagResponseWrapper extends HttpServletResponseWrapper {

		private final ByteArrayOutputStream content = new ByteArrayOutputStream();

		private final ServletOutputStream outputStream = new ResponseServletOutputStream();

		private PrintWriter writer;

		private ShallowEtagResponseWrapper(HttpServletResponse response) {
			super(response);
		}

		@Override
		public ServletOutputStream getOutputStream() {
			return this.outputStream;
		}

		@Override
		public PrintWriter getWriter() throws IOException {
			if (this.writer == null) {
				String characterEncoding = getCharacterEncoding();
				this.writer = (characterEncoding != null ? new ResponsePrintWriter(characterEncoding) :
						new ResponsePrintWriter(WebUtils.DEFAULT_CHARACTER_ENCODING));
			}
			return this.writer;
		}

		@Override
		public void resetBuffer() {
			this.content.reset();
		}

		@Override
		public void reset() {
			super.reset();
			resetBuffer();
		}

		private byte[] toByteArray() {
			return this.content.toByteArray();
		}

		private class ResponseServletOutputStream extends ServletOutputStream {

			@Override
			public void write(int b) throws IOException {
				content.write(b);
			}

			@Override
			public void write(byte[] b, int off, int len) throws IOException {
				content.write(b, off, len);
			}

		}

		private class ResponsePrintWriter extends PrintWriter {

			private ResponsePrintWriter(String characterEncoding) throws UnsupportedEncodingException {
				super(new OutputStreamWriter(content, characterEncoding));
			}

			@Override
			public void write(char buf[], int off, int len) {
				super.write(buf, off, len);
				super.flush();
			}

			@Override
			public void write(String s, int off, int len) {
				super.write(s, off, len);
				super.flush();
			}

			@Override
			public void write(int c) {
				super.write(c);
				super.flush();
			}

		}

	}

}
